1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel strip for use as an iron core of electrical equipment. In particular, the present invention is concerned with a process for producing a grain-oriented electrical steel strip having a very high magnetic flux density through studies and optimization of conditions for annealing of a hot-rolled strip after hot rolling in a production process wherein a steel slab is heated at 1200.degree. C. or below, that is, a production process wherein an inhibitor is formed in situ after the completion of cold rolling in a one-stage cold-rolling process or two-stage cold rolling process.
2. Description of the Prior Art
A grain-oriented electrical steel strip is produced so as to exhibit an excellent magnetic property only in the direction of rolling, and can be used to produce a transformer having excellent performance. The grain-oriented electrical steel strip is characterized by a secondary recrystallized grain from the viewpoint of the growth of a crystal. In order to accelerate the growth of the secondary recrystallized grain, it is necessary to regulate the growth of a primary recrystallized grain through the addition of a very small amount of an inhibitor element. For example, in a two-stage cold rolling process, in many cases, MnS is used as the inhibitor. In general, this process comprises adding Mn or S in the step of producing a steel, hot-rolling the steel, cold-rolling the hot-rolled steel twice with intermediate annealing being conducted between the cold-rollings into a strip having a final thickness and subjecting the strip to decarburization annealing and final annealing to grow a crystal grain.
In one-stage the cold rolling process, in many cases, AlN is used as the inhibitor. In this process, conditions for the inhibitor are important, and regulation is conducted so that the growth of the primary recrystallized grain is prevented while the secondary recrystallization is accelerated. Specifically, in the one-stage cold rolling process, it is known that in order to obtain a secondary recrystallized grain having a higher degree of pole concentration, the inhibitor should exhibit a stronger restraint than that in the case of the two-stage cold rolling process for the purpose of suppressing the growth of a primary recrystallized grain having a smaller size derived from a high rolling reduction and, at the same time, conducting the formation and growth of a secondary recrystallization nucleus.
A grain-oriented electrical steel strip is used mainly as an iron core material for a transformer, a generator and other electrical equipment. A high magnetic flux density, a watt loss and a magnetostriction at an ordinary frequency are important properties required of the grain-oriented electrical steel strip. The magnetic flux density is determined by the degree of pole concentration of {110}&lt;001&gt; orientation. Further, the grain-oriented electrical steel strip should have excellent magnetic properties, that is, a magnetization property and a watt loss property, and further should have a good coating.
The grain-oriented electrical steel strip can be prepared by selectively evolving a crystal grain having the so-called "Goss texture", that is, having a {110} plane on the rolled plane and a &lt;001&gt; axis in the direction of rolling through the utilization of a secondary recrystallization phenomenon.
As is well known in the art, the secondary recrystallization occurs in finish annealing after decarburization annealing subsequent to cold rolling. In order to satisfactorily form the secondary recrystallization, the growth of the primary recrystallized grain should be inhibited as much as possible until the temperature reaches a secondary recrystallization region. For this reason, fine precipitates such as AlN, MnS and MnSe, that is, inhibitors should be present in the steel.
Therefore, in the process for producing an electrical steel, an electrical steel slab is heated at a high temperature of 1350.degree. to 1400.degree. C. for completely dissolving an inhibitor forming element added in the stage of making a steel, for example, Al, Mn, S, Se or N. Thus, the inhibitor forming element completely dissolved in a solid solution form in the electrical steel slab finely precipitates as AlN, MnS and MnSe through intermediate annealing at the stage of a hot-rolled strip after hot rolling or an intermediate gauge before the final cold rolling.
With respect to prior art relevant to the above-described process, Japanese Examined Patent Publication (Kokoku) No. 46-23820 discloses a method for precipitating AlN having a preferred size in the steel strip which comprises incorporating C and Al in a common steel or a silicon steel to form a secondary recrystallized grain having a {110}&lt;001&gt; orientation, wherein annealing immediately before the final cold rolling is conducted at a temperature of 750.degree. to 1200.degree. C. and quenching is conducted at a temperature of 750.degree. to 950.degree. C. depending upon the amount of Si. Japanese Unexamined Patent Publication (Kokai) No. 50-15727 discloses a process for producing a grain-oriented electrical steel strip which comprises hot-rolling a steel containing C, Al, Mn, N, Cu or the like and cold-rolling the steel at least once, wherein, before the final cold rolling, the steel strip is annealed at a temperature of 760.degree. to 1177.degree. C. for 15 sec to 2 hr and cooled from a temperature of 927.degree. C. or less and 400.degree. C. or above to a temperature of about 260.degree. C. or below at a rate higher than a natural cooling rate.
These method can be applied only to the material which is hot-rolled after completely dissolved a fine precipitation by raising a heating temperature of a steel slab.
In such a process, as described above, since the electrical steel slab is heated at a high temperature, the amount of occurrence of a molten scale (slag) during the heating is so large that the frequency of repair of the heating furnace becomes high. This gives rise to problems such as an increase in the maintenance cost, a lowering in the operation rate of facilities and an increase of fuel consumption in unit of steel. Studies have been made on a process for producing a grain-oriented electrical steel strip having excellent properties wherein an electrical steel slab is heated at a lowered temperature. For example, Japanese Examined Patent Publication (Kokoku) No 61-60896 discloses a process which comprises heating a material comprised of an electrical steel slab having a Mn content of 0.08 to 0.45%, a S content of 0.007% or less, a lowered value of the product [Mn][S] and, incorporated therein, Al, P and N at a temperature of 1200.degree. C. or below. And Japanese Unexamined Patent Publication (Kokai) No. 1-230721 discloses the same process which comprises heating an electrical steel slab containing Al, N, B, Ti or the like at a temperature of 1200.degree. C. or below. In recent years, as opposed to the above-described process wherein an inhibitor is formed in situ through a solution heat treatment at a high temperature before the step of cold rolling, a process wherein the inhibitor is formed in situ in a step after the cold rolling has been developed. This has enabled a grain-oriented electrical steel strip having excellent properties to be produced through the regulation of the texture (recrystallization ratio, transformation phase, etc.) alone in the steps of hot rolling and annealing of hot-rolled strip.
As well known in the art, the secondary recrystallization phenomenon occurs during finish annealing after decarburization annealing subsequent to cold rolling. In order to satisfactorily form the secondary recrystallization, the growth of the primary recrystallized grain should be inhibited as much as possible until the temperature reaches a secondary recrystallization region. For this reason, fine precipitates such as AlN, MnS and MnSe, that is, inhibitors, should be present in the steel.
The present invention provides a process for producing a grain-oriented electrical steel strip having a very high magnetic flux density through studies and optimization of conditions for annealing of a hot-rolled strip after hot rolling in a production process wherein a steel slab is heated at 1200.degree. C. or below, that is, a production process wherein an inhibitor is formed in situ after the completion of cold rolling in one-stage cold-rolling process or two-stage cold rolling process.